(1) Field of the Invention
The present invention pertains to a plate for joining broken bones, and in particular, a plate designed to rigidly fix two bone parts in the correct anatomical position so as to permit the broken or disjointed bones to correctly heal.
(2) Prior Art
The use of internal osteosynthetic fixation plates and screws for long-bone fractures has been known as early as 1886. Many improvements have occurred over the years as a result of the direct observation of the progress of bone healing during the employment of fixation plates.
In the early stages of fracture healing with a fixation plate, the fracture site is exposed to eccentric axial forces which may result in high bending stresses in the fixation device that remain present until solid fracture healing is achieved. Especially high bending stresses may be encountered in the bone with severely comminuted fractures or in delayed unions having poor bone contact. High bending forces in the fracture fixation plate may result in motion at the fracture site and possible fatigue failure of the fixation plate before bone union is achieved. Because of this possible complication it is preferabe to utilize a device which has high bending stiffness and strength. To obtain better torsional stability at the fracture site, it is also preferable to use a device which has a large torsional rigidity and torsional strength. Also, in the early stages of fracture healing, the vascular demands of the injured bone are great, and, in order to promote more rapid fracture healing, the vascular supply to the fracture site should be disturbed as little as possible. In the case of bone plating, this can be achieved by using a more narrow plate particularly at the fracture point.
In the late stages of fracture healing, the bone is capable of sharing the axial forces with the fixation plate. However, if the actual stiffness of the plate is high, the plate will "overprotect" the bone and the bone will become osteoporotic, making it more susceptible to refracture after plate removal. This complication may be avoided by the use of a fixation plate with low axial stiffness. Thus, a perfect fixation plate has high bending stiffness and strength, large torsional rigidity and torsional strength, and low axial stiffness.
Fracture fixation plates which are now known and in use fail to meet the requirements of bone fracture mending in a number of ways. The axial stiffness or rigidity of an implant device is proportional to its cross-sectional area and therefore directly proportional to plate thickness and width. The bending rigidity and strength are proportional to a power function of the thickness. Since conventional plates are much wider than they are thick, they have a high axial stiffness and strength and relatively poor bending stiffness and strength. In addition, the width of the plate may significantly disturb the vascular supply to the fracture site. The following patents are illustrative of prior art devices.
French Pat. No. 1,051,847 issued to Carrieri discloses a bone fixation plate comprising three members, namely, two identical base members and an internally threaded union element. Each base member includes a portion which contacts the bone. At one end of each base member there is an upwardly extending, non-contacting threaded projection. Each base member is attached to the bone on opposite sides of the fracture site in axial alignment with one another. The internally threaded union member is then threaded onto each of the threaded projections which face one another to draw each base member together, and thus the corresponding bone pieces toward one another, to assure good contact between the bone pieces at the fracture site. This device has poor bending rigidity because of its wide width and small thickness, and additionally, there is some sloppy movement because the stress is concentrated at the threads.
U.S. Pat. No. 3,528,085 to Reynolds, Jr. discloses a one-piece bone compression plate having a plurality of screw holes. The pair of holes which straddle the fracture site are elongated and include a sliding ramp. When the bone compression plate is properly positioned with screws inserted in each elongated hole straddling the fracture site, the bone is placed in compression at the fracture site by tightening the screws forcing them to slide down their respective ramps thereby drawing the bone pieces toward one another and placing them in compression. The bone compression plate has a large uniform width and a small constant thickness. Thus it disturbs the vascular supply to the fracture site and has poor bending rigidity and strength at the fracture site since the plate is greater in width than in thickness.
U.S. Pat. No. Re. 28,841 to Allgower, et al discloses an osteosynthetic pressure plate having a plurality of elongated screw holes at each end of the pressure plate. Each screw hole has an oblique portion designed to be engaged by the underside of a semispherical head of a bone-fixing screw. The plate secured with screws will hold the bone parts in compression by drawing each bone part toward one another in tight engagement. The bone-fixing screws must have a head whose underside is semispherical in form so as to slide down the oblique portion of each elongated screw hole of the pressure plate. Additionally, the bone fixation pressure plate has a large uniform width and a small uniform thickness so that no increase in bending strength is obtained. Also, the vascular supply at the fracture site is reduced since the compression plate has a large uniform width.
French Pat. No. 2,367,479 to De Bazelaire discloses a bone fixation plate which is thicker at the center and gradually tapers toward each end thereof. This plate has a uniform width, and the underside thereof may be curved to more fully correspond with the shape of the bone. While this plate does have good bending rigidity and strength, the plate has high axial stiffness and prevents good vascular supply to the fracture site since the width of the plate is constant.